Smart windows block heat while generating electricity

Scientists at the University of California Berkeley have made a major breakthrough by developing windows that automatically tint on a sunny day and which can also convert the sunlight into electricity. Elke Bunge reports.

Researchers in the U.S. have combined tinted windows with a solar cell that generates electricity when darkened. (Image credit: University of California)

Windows that tint at the touch of a button are set to make curtains and blinds a thing of the past. They keep working or living spaces pleasantly cool, especially in hot climates or during the sweltering summer months – a mechanism which one its own saves on sizeable energy costs that would otherwise be used for an air conditioning system.

Scientists at the University of California Berkeley have now taken this one step further combining tinted windows with a solar cell that generates electricity when darkened.

Switching solar cells on and off

The bright sunlight causes the window to automatically tint, blocking the heat from entering the room while at the same time converting the sunlight into electricity.

“This smart photovoltaic window can be switched between a transparent state and a non-transparent, photoactive state,” explained Jia Lin from the Department of Chemistry at the University of California Berkeley.

Window coated with perovskite

For their prototype, Lin and his colleagues coated a transparent glass substrate with a 200-nanometre-thin layer of perovskite.

Perovskite, a mineral found in abundance in nature, is one of the most promising materials in solar research at this time. Its positive properties are based on its crystal lattice: when a light particle hits it, it excites an electron. When the electron returns to its basic state, energy is released that can be used to generate electricity.

In other words: it can be used to “harvest” sunlight.

Generating electricity at high temperatures

And this “harvesting” would appear to be highly effective: the glass window with the thin-film perovskite surface allows more than 80 per cent of visible light to pass through at room temperature. This corresponds to the transparency of a conventional glass pane.

But when the surface is exposed to temperatures of more than 100 degrees Celsius, crystal structure changes and the perovskite turns orange-red. When this happens, the perovskite absorbs two-thirds of the sunlight and becomes photoactive. The incident light can now be used to generate electricity.

This smart photovoltaic window has an efficiency of just over 7 per cent, which means the solar cell is able to convert 7 per cent of the incident solar energy into electricity. For a perovskite solar cell, the figure is 22 per cent.

Skyscrapers produce their own electricity

An efficiency of 7 per cent can be improved on. What’s more important is that the researchers have succeeded in developing a combination of window and solar cell.

The major advantage of this development lies in its potential applications: in the future, megacities could easily supply themselves with renewable energy, opening up new possibilities for cities like Tokyo, Seoul, Mexico City or New York, each of which are home to nearly 10 million people, as well as other major cities and their sea of skyscrapers.

Large cities generating their own electricity from the power of the sun; it may not be science fiction, but it is certainly the dream of many a futurist.

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